Raman optical amplifiers are important components in optical communication systems. Optical fiber communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical fiber system comprises a source of information-carrying optical signals, an optical fiber transmission line for carrying the optical signals and a receiver for detecting the optical signals and demodulating the information they carry. The signals are typically within a wavelength range favorable for propagation within silica fibers, and preferably comprise a plurality of wavelength distinct channels within that range.
Despite significant progress in reducing the attenuation characteristics of optical fibers, signals transmitted through them are attenuated by the cumulative and combined effect of absorption and scattering. Consequently long distance transmission requires periodic amplification.
One approach to optical amplification utilizes Raman effect amplification. In the Raman effect amplification, light traveling within a medium is amplified by the presence of lower wavelength pump light traveling within the same medium. The gain spectrum of a silica fiber pumped by a monochromatic Raman pump exhibits maximum gain when the signal to be amplified is at a frequency approximately 13 THz lower than the frequency of the Raman pump. The frequency (or wavelength) difference between the pump and the frequency (or wavelength) of maximum gain is often referred to as the Stokes shift, and the amplified signal is referred to as the Stokes wave. Use of a pump that is detuned from the signals by about one Stokes shift (1/2 the Stokes shift to 3/2 the shift) is referred to as first-order Stokes pumping.
It has also been observed that the gain is significantly larger for a co-polarized signal and pump. This polarization sensitivity can be eliminated if the pump is depolarized, polarization-scrambled or composed of two equally powerful polarized pumps that are polarization multiplexed. See, for example, U.S. Pat. No. 4,805,977, issued to Y. Tamura et al and entitled "Optical Coupler for Optical Direct Amplifier".
Raman amplifiers can be categorized as either distributed or discrete. In distributed amplifiers, the transmission fiber itself is used as the gain medium. In discrete amplifiers, a separate fiber, typically optimized for Raman amplification, is used as the gain fiber. While the discrete amplifier gain fiber may be kilometers in length it is typically spooled at one location and not used to transfer information from one location to another. The term "Raman amplifier", as used herein, refers to both the pump and the gain medium.
Signal amplification utilizing distributed first order Raman effect amplifiers is described in U.S. Pat. No. 4,616,898 issued to John W. Hicks, Jr. on Oct. 14, 1986. The Hicks et al. system disposes a plurality of optical Raman pumps at spaced intervals along the transmission line. These pumps inject pump light into the optical fiber at a wavelength shorter than the signal wavelengths by a Stokes shift, so that the pump light amplifies the lower wavelength signals by the first order Raman effect.
A difficulty with conventional Raman amplifiers is that they are typically critically dependent on power sensitive components subsequent (downstream) to the pump. Often Raman pump sources utilize a plurality of sources to establish a high power first order pump (&gt;100 mW) and an immediate downstream multiplexing component to combine the outputs into a pump with a wide, flat bandwidth. In typical amplifiers, the pump power is generated by an array of high power semiconductor pump lasers that are followed by wavelength-division multiplexers. See, for example, Y. Emori et al., "Less than 4.7 dB Noise Figure . . . ", Conference on Optical Amplifiers, Paper PD3-1, Vail Colorado (Jul. 27-29, 1998). The problem with such amplifiers is that the multiplexers present relatively high insertion loss to the high power signals. Moreover the multiplexers are not reliable at high power levels. Other Raman pump sources utilize plural sources of different polarization followed by polarization multiplexers. The polarization multiplexers present the same problems of insertion loss and unreliability at high power. And even single source high power pumps are sometimes followed by lossy or unreliable components as polarization scramblers or spectral filters. Accordingly there is a need for an improved Raman amplifier with reduced sensitivity to power sensitive components.